In many cases it is desired to indicate on a tube the correct position of installation of the tube. Such indication may be necessary for example in case of infiltration tubes having slots which are asymmetrically distributed; i.e. they are arranged only on that portion of the tube wall, which shall face downwards when the tube is positioned in the ground, i.e. that part of the tube wall, which then shall form the bottom portion of the tube. The indication at present is provided by applying to the tube a longitudinal colour strip which is produced by means of a spray gun or tape or by extrusion from a separate side extruder. In case of plastic tubing having a corrugated or finned outside surface the last-mentioned method is the only one that can be used. However, this method involves high investment cost for a supplementary extruder and moreover causes some operating problems if the tube is a double-walled tube of the type having a corrugated outer wall and a smooth inner wall, because two separate extruders are required for the two walls.
The method of the invention has been developed for such tubes and other plastic tubes having two layers which are extruded from plastic materials of different colours for the inner layer and the outer layer no matter how the two layers are formed, in order to provide in a very simple manner an indication of the correct position of installation by means of a strip on the outer layer, which has a colour deviating from the colour of the remaining portion of the outer layer.
The invention also relates to a tool for working the method.